This invention is directed to TM.sub.0i0 mode (i=1, 2, 3, . . . ) circular and polygon shaped planar high temperature superconductor resonators having a hole in the center thereof, and their applications to high temperature superconducting filters, filter banks and multiplexers.
Filter banks and multiplexers are widely used in telecommunications as channelizers, which separate or combine the incoming signals according to their frequencies. The basic building block of filters banks and multiplexers is a filter, which comprises a number of resonators as the frequency selecting element. For applications in telecommunication, the filters need to have narrow bandwidth, accurate center frequency, low insertion loss in the in-band, high rejection in the off-band, steep skirts at the edges of the in-band, compact size and high power handling capability. Conventional filters made from normal conductors are not well suited for telecommunication uses because of the high loss in the normal conductors.
High temperature superconductors (HTS) planar filters have been made with excellent performance at low power. See Zhi-Yuan Shen, High Temperature Superconducting Microwave Circuits, Artech House, Boston, 1994, p. 113. While these HTS planar filters have utility in receivers, due to their very limited power handling capability, they are not well suited for use in transmitters. For application in transmitters, the filters must handle power ranging from ten watts to hundreds of watts. In commonly assigned, U.S. Pat. No. 5,710,105, we disclose TM.sub.0i0 mode (i=1, 2, 3, . . . ) circular and polygon shape HTS filters, filter banks and multiplexers which are capable of handling more than 100 watts of transmitting power.
The center frequency accuracy is another important requirement, particularly for a narrow band filter. This is especially true for the filters in the so called "contiguous" multiplexer, such as that described in Zhi-Yuan Shen, supra at page 120, and in a multi-pole filter, where loss of center frequency accuracy will severely degrade performance. Unfortunately, the frequency of HTS resonators in a filter may deviate from the design value due to circuit fabrication tolerances and the uncontrollable variations in the substrate such as the thickness changes or "twin boundary." See Zhi-Yuan Shen, supra at 12.
In commonly assigned, copending application Ser. No. 08/227,437, filed Apr. 14, 1994, now abandoned, we disclose planar HTS filters in a "stacked form", in which the individual HTS resonators are stacked vertically and coupled via holes or slots in the ground plates. Coupling, however, is only provided between adjacent resonators. Certain type of filters, such as the "elliptical frequency response" bandpass filter, require "jump-over" coupling, that is, coupling between resonators that are spaced from each other by one intermediate resonator.